The invention relates to control systems for the start circuit switch of a single phase AC induction motor.
The invention particularly arose from efforts to provide a low cost start or disconnect switch, including replacement of a high cost solid state switch with a pair of lower cost SCRs in inverse parallel, though the invention is not limited thereto. A single triac conducting current in both directions is more expensive than a pair of SCRs connected in inverse parallel. Furthermore, a triac affording a higher voltage blocking capability than individual SCRs is also more expensive. The present invention reduces cost by enabling a pair of lower rated inverse parallel connected SCRs, by providing an auxiliary switch rapidly discharging the start capacitor upon turn-off of the motor start circuit when the motor reaches cut-out speed. This removes any additional voltage across the SCRs otherwise caused by the charged start capacitor, and hence allows the use of lower rated reverse breakdown SCRs and also extends the life of the start capacitor.
The invention represents continuing development and improvement over commonly owned co-pending: U.S. application Attorney Docket No. 06/910,931, filed herewith, "Low Cost, Protective Coast To Restart Circuit", Lorenz et al; U.S. application Ser. No. 06/885,087, filed July 14, 1986, "Low Cost, Load And Speed Sensitive Motor Starting Circuit", Wrege et al; and allowed U.S. application Ser. No. 680,489, filed Dec. 11, 1984, "Load And Speed Sensitive Motor Starting Circuit", Shemanske et al, all hereby incorporated herein by reference.